Method of making hot mill tools



Patented Nov. 27, 1951 METHOD OF MAKING HOT MILL TOOLS Paul L. Daley and Stanley N. Lutz, Ellwood City, Pa., assignors to National Tube Company, a corporation of New Jersey No Drawing. Application August 31, 1948,

I Serial No. 47,138

1 Claim. (01. 148-635) The present invention relates to heat treated alloys having physical and chemical properties making them desirable for use as hot mill tools which are subjected to high compressive and frictional loads at elevated temperatures.

In the production of seamless tubes, special tools are required on the piercing and rolling mills to produce tubes of satisfactory surface and dimensions. Certain of these tools, notably guide shoes, piercer points and plugs, require steel that possesses excellent wearresistance and resistance to deformation and softening at the high temperatures encountered during the rolling operations. In the same category fall hot draw dies for shell manufacture, mill wearing plates and. wear resisting inserts on equipment. subjected to compressive loads at high temperatures. Such hot mill tools are commonly made from high alloy steels having relatively high carbon content.

By using steel of the following composition treated as hereunder described, we have produced hot mill tools having the desired qualities of wear resistance, resistance to heat checking and hardness at high temperatures. Broadly, an alloy suitable for such tools may contain between about .50% and about 1.75% of carbon, between about 13.00% and 18.00% of chromium, between about 2.00% and about 6.00% of nickel, between about 20% and about 1.00% of manganese, between about .80% and about 3.00% molybdenum, about 1.00% maximum of silicon, about .05% maximum each of phosphorus and sulphur, the remainder being iron and incidental impurities. The molybdenum promotes toughness and wearresistance and aids in producing smaller carbides and also reduces heat checking in service. With higher molybdenum concentrations, nickel in the lower concentrations can be employed. The chromium and nickel give high temperature strength and toughness and certain other qualities hereinafter discussed.

A preferred composition will have the alloying elements within the following ranges:

Per cent Carbon .70 to 1.25 Chromium 15.00 to 17.00 Nickel 3.00 to 4.00 Manganese .30 to .60 Molybdenum .80 to 1.25 Silicon 1.00 maximum Phos horus .05 maximum Sulphur .05 maximum A carefully controlled heat treating procedure is necessary to adapt the foregoing alloys for use as hot mill tools. The alloy is preferably cast by conventional foundry practice. The castings after being ground to size are austenitized by being reheated to about 21002200 F. for sufficient time to dissolve a portion of the carbides present in the as cast state preferably for about one hour and then air cooled to at least l 000 F. structurally, the alloys are dentritic as cast and are substantially austenitic with excess carbides outlining the dentritic grains. The reheating step does not break up the dentrites but does dissolve a portion of the carbides present in the as cast structure.

excess carbides. v

' A scale of a particular type is produced on castings of the foregoing composition by the austenitizing treatment which scale is resistant to the effects of heat and pressure encountered in use by hot mill tools. The abrasive action incident to the friction of the tools against hot metal workpieces results in wear. However, as the scale 'wears off the tools of our invention, it is constant- 1y renewed due to heat transfer from the workpiece. The rate of formation as well as character of .the scale renders the tools of our invention particularly successful in use. 'The chromium and nickel contents must be closely controlled within the foregoing limits to obtain the desired scaling eifect.

An important aspect of this heating step is that with the alloy of our invention, it has been found that with further heat treatment in the temperature range of about 1300 to 1800 F. for about two to eight hours, there takes place an unexpected increase in hardness of from about 250 B. H. N. to about 400 to 500 B. H. N. with formation of a martensitic structure. Reheating to various temperatures, from 1200 up to 1900 F., after the austenitizing treatment produces a decomposition of the austenite which is apparent from the structure and physical properties of the alloy as well as from the fact that it becomes magnetic after such treatment.

After reheating to 1100 F. there occurs no apparent structural change although the hardness increases slightly. This change is ascribed to the occurrence of submicroscopic precipitation. After reheating to 1200 F. the hardness is the same as after 2200 F. treatment and samples previously treated at 1100 F. are reduced in hardness by treating at 1200 F. This phenomenon is ascribed to a coalescence or slight growth of the submicroscopic particles and a relief of in- When air cooled from about 2100 to 2200 F. many of the carbides remain in solution and the structure is essentially austenitic withternal strain resulting from initial precipitation and cooling.

Further decomposition of the austenite occurs as the reheating temperature is raised from 1300 to 1800 F. The hardness also increases to a maximum of about 500 B. H. N. after a 1700 F. reheat/- Gui-ch at n fr l3QQ E'- E0 ab ut 1900 F. the hardness decreases to approximately that of the 2200 F. treated samples and magnetism disappears.

After reheating from 1300 to 1800 F. the structure appears metallographically to be fine, magnetic, alloy martensite containingthe. sigma phase throughout its matrix with excess carbides outlining the original dendritic grains. Particularly successful castings of the foregoing alloys treated as described have been used for piercer guide shoes, high mill plugs and hot draw dies for shell manufacture.

A further feature of the hot mill tools is that they can be hardened to a definite hardness in the second heat treatment despite variations in hardness after the first heat treatment. Due to difierences in composition, temperature of the first heat treatment and rate of coolingthe hardness may vary from 250. to 350 B. H. N. How.- ever, the nature of this alloy is such that for any given hardness within the foregoing range, the second heat treatment can be made. to yield any desired hardness offrom 400 to 500 B. H, N. regardless if variations in chemistry and initial treatment by varying the temperature and time 0i reatment. This a part cularly desirable eature in ols. suchas hi h mill teels which require varying degrees of hardne s. tor optimum results.

Heretofore, the scale formed on heat treating has ordinarily been removed before use..

However, this is not only not necessary but. is undesirable with the tools of the present inven-: tion. As before stated, the particular alloy and heat treatment discovered by us produce a beneficial hard adherent film of oxide scale that acts as a lubricant producing excellent operating results and long life of the tools. The long life is, further enhanced by the ability of the scale to keep rebuilding or regenerating itself during the hot work process. Unusual operating conditions at times cause fracture and spalling of the heat treatment scale. When this occurs, it is advisable to replace the plugs with new plugs having an intact coating of the beneficial scale thereon. The scales which form on various alloys when subjected to heating vary widely in their characteristics. In many cases, the scale is brittle and flakes ofi readily. However, the scale that forms on the present alloys is a tough, adherent, continuous coating which is resistant to the effects of heat and pressure encountered by 'tools for working hot metal. The present scale forms a protective glaze which prevents metal-to-metal contact with subsequent elimination of galling and seizin n use.

Weclairn: V V

A method of forming hot mill tools comprising forming said tools of a steel containing .70 and 1.25% of carbon, '15 and 17% of chromium, 3 and'4% of nickel, .30 and .60% of manganese, .80 and. 1.25% of molybdenum, with the balance substantially iron, said steel being initially austenitic, heating said steel'in the temperature range of 2100 to 2200 F. under oxidizing conditions for about one hour to dissolve a portion of the carbides and develop a dense adherent scale thereon which acts as a self renewing lubricant self-renewing during use, cooling it to at least 1000 F. and then reheating it to 'a temperature between 1300 and 1800 F. for two to eight hours to render said steel magnetic and impart a hardness thereto of between 400 and 500 B. H. N.

' PAULL. DALEY.

STANLEY. N. LUTZ.

BEFERENCES CITED The following references are of r cord in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,375,255 MacGregor et al. Apr. 19, 1921 1,963,319 Wright June 19, 1934 2,009,074 Payson July 30, 1935 2,197,098 Davis et al. Apr. 16, 1940 2,479,579 Spence et a1. Aug. 23, 1949 OTHER REFERENCES Stainless Iron and Steel, by Monenpenny, pages 2 -229. 1931. 

